This invention relates to the use of water-soluble polymers for inhibiting formation of gas hydrates in pipes containing oil or gas. This is relevant for both drilling and production of oil and gas.
Gas hydrates are clathrates (inclusion compounds) of small molecules in a lattice of water molecules. In the petroleum industry natural gas and petroleum fluids contain a variety of these small molecules which can form gas hydrates. They include hydrocarbons such as methane, ethane, propane, isobutane as well as nitrogen, carbon dioxide and hydrogen sulphide. Larger hydrocarbons such as n-butane, neopentane, ethylene, cyclopentane, cyclohexane and benzene are also hydrate forming components. When these hydrate forming components are present with water at elevated pressures and reduced temperatures the mixture tends to form gas hydrate crystals. For example, ethane at a pressure of 1 MPa forms hydrates only below 4xc2x0 C. whereas at 3 MPa gas hydrates can only form below 14xc2x0 C. These temperatures and pressures are typical operating environments where petroleum fluids are produced and transported.
If gas hydrates are allowed to form inside a pipe used to transport natural gas and/or other petroleum fluids they can eventually block the pipe. The hydrate blockage can lead to a shutdown in production and significant financial loss. The oil and gas industry uses various means to prevent the formation of hydrate blockages in pipelines. These include heating the pipe, reducing the pressure, removing the water and adding antifreezes such as methanol and ethylene glycols which act as melting point depressants. Each of these methods is costly to implement and maintain. The most common method used today is adding antifreezes. However, these antifreezes have to be added at high concentrations, typically 10-40% by weight of the water present, in order to be effective. Recovery of the antifreezes is also usually required and is a costly procedure.
Consequently, there is a need for alternate cheap methods for preventing hydrate blockages in oil and gas drilling and production.
An alternative to the above methods is to control the gas hydrate formation process using nucleation and crystal growth inhibitors. These types of chemicals are widely known and used in other industrial processes. The advantage of using these chemicals to control gas hydrate formation is that they can be used at concentrations of 0.01 to 2% which is much lower than for antifreezes.
It is an object of this invention to provide an additive and a method of controlling gas hydrate formation using said additives added at low concentrations to a stream of at least some light hydrocarbons and water.
According to the present invention we provide the use of polymers which comprise structural elements of the formula 
wherein
each R is independently H or C1-C5-alkyl;
X is H, an alkaline or earth alkaline metal or a quarternary ammonium group;
R1 is H or C1-C18-alkyl; and
R2 is C1-C18-alkyl;
and wherein the alkyl groups represented by R1 and R2 may carry a hydroxy or amino substituent;
and, if desired, a minor proportion of structural elements of the formula 
xe2x80x83wherein R1, R2 and X are as above, and Alk is a C1-C5-alkylene chain, and, if desired, also other structural elements formed from ethylenically unsaturated monomers;
the molecular weight of the polymer being in the range from 500 to 2,000,000, as an additive for inhibiting the formation of gas hydrates in connection with hydrocarbon production and transportation.
When reference is made to formula I in the following, this may also include minor amounts of II.
The polymers preferably have a molecular weight in the range 1000-1,000,000. The units of formula I may be different, and there may also be other units which are different from formula I. Such other units may be present in the polymer in amounts up to 90% of the polymer based on the total number of units in the polymer. Sometimes it may be advantageous to have as little as 1% of such other units in the polymer. A polymer having units of formula I and said other units in a ratio of 2:1 to 1:2 may also be preferred. The distribution of the units in the polymer may be random or an exact alternation (in particular when the ratio is 1:1).
The polymer can contain more monomers giving rise to units of formula I in a polymer formed by reaction of one or more primary or secondary amines having 1-18 carbon atoms with polymers or copolymers of maleic anhydride. Additionally the polymer can be made by reacting one of more monoamines having 1-18 carbon atoms and one or more hydroxyamines with polymers or copolymers of maleic anhydride. The polymer can be a homopolymer or a copolymer with other ethylenically unsaturated monomers including alkyl vinyl ethers, (meth)acrylates, hydroxyalkyl(meth)acrylates, vinyl carboxylates, alkenes, vinyl lactams, vinyl amides, acrylamidopropylsulphonic acid (AMPS), vinylsulphonic acid, alkyl(meth)acrylamides, styrene, allyl amides, vinylphosphoric acid and styrenesulphonic acid.
Instead of amidating the maleic anhydride polymer it is also possible to amidate the corresponding maleic anhydride to form a compound of the formula 
wherein
each R is independently H or C1-C5-alkyl;
X is H, an alkaline or earth alkaline metal or a quaternary ammonium group;
R1 is H or C1-C18-alkyl, hydroxyalkyl or aminoalkyl; and
R2 is C1-C18-alkyl, hydroxyalkyl or aminoalkyl.
This monomer may then be subjected to polymerisation, if required together with a comonomer.
Examples of alkylamines that can be reacted with maleic anhydride and polymers thereof to form the desired product include methylamine, dimethylamine, ethylamine, diethylamine, n-propylamine, iso-propylamine, iso-butylamine and n-butylamine.
Examples of hydroxyamines that can be added to the reaction mixture of alkylamine and maleic anhydride polymers include 2-amino-2-methyl-1-propanol, 2-aminoethanol, 2-(2-aminoethylamino)ethanol, 2-(2-aminoethoxy)ethanol, dimethylethanolamine, 3-(dimethylamino)-1-propanol, 1-(dimethylamino)-2-propanol, N,N-dibutylethanolamine and 1-amino-2-propanol as well as polyglycols of ethylene oxide, propylene oxide and butylene oxide having one amine end group.
When a hydroxydialkylamine such as 3-(dimethylamino)-1-propanol is used, the reaction with the maleic anhydride groups will always result in structural elements of formula II since a disubstituted amino group cannot react with the maleic anhydride.
Examples of alkyl diamines which can be added to the reaction mixture of alkylamine and maleic anhydride polymers include 3-dimethylaminopropylamine and 3-diethylaminopropylamine.
At least one of the alkylamines to be reacted with maleic anhydride polymers is preferably chosen from C3-C4-alkylamines, in particular n-propylamine, iso-propylamine, n-butylamine and isobutylamine. Thus, one of R1 or R2 is preferably n-propyl, iso-propyl, n-butyl or iso-butyl.
Two or more amines can be reacted with the maleic anhydride polymer to increase performance or for compatibility with the aqueous phase. Two examples to illustrate this but which are not meant to limit the scope of application include a mixture of isobutylamine and a hydroxyamine or a mixture of isobutylamine and methylamine.
The amidated maleic anhydride monomers can be structurally part of copolymers comprising other comonomers such as alkenes; alkyl vinyl ethers, (meth)acrylates, hydroxyalkyl(meth)acrylates, vinyl carboxylates, vinyl lactams vinyl amides, acrylamidopropylsulphonic acid (AMPS), vinylsulphonic acid, alkyl(meth)acrylamides, styrene, allyl amides, vinylphosphoric acid and styrenesulphonic acid. Examples of alkenes include 1-alkenes having 2-24 carbon atoms and iso-butylene.
Examples of (meth)acrylates include acrylic acid and acrylate salts, methacrylic acid and salts, C1-24 alkyl acrylates, C1-24 alkyl methacrylates, dimethylaminoethyl(meth)acrylate and trimethylammonium-ethyl(meth)acrylate chloride.
Examples of hydroxyalkyl(meth)acrylates include hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate and polyglycol esters of acrylic acid.
Examples of alkyl vinyl ethers include methyl vinyl ether and isobutyl vinyl ether.
Examples of vinyl carboxylates include vinyl acetate.
Examples of N-vinyl lactams include N-vinylcaprolactam, N-vinylpiperidone and N-vinylpyrrolidone.
Examples of vinyl amides include N-vinylacetamide, N-vinyl-N-methyl-acetamide and N-vinylformamide.
Examples of alkyl(meth)acrylamides include acrylamide, methacrylamide, N-methylacrylamide, N,N-dimethylacrylamide, N-ethylacrylamide, N,N-diethylacrylamide, N-isopropylacrylamide, N-isopropylmethacrylamide, N-isobutylacrylamide, acryloylpyrrolidine, methacryloylpyrrolidine, N-octyl-acrylamide, stearylacrylamide, N-methylol(meth)acrylamide, N-butoxymethyl(meth)acrylamide, N-isobutoxymethyl(meth)acrylamide, dimethylaminopropyl(meth)acrylamide and trimethylammoniumpropyl(meth)acrylamide chloride.
Depending on the chemical structure of the comonomers, the effect of the resulting polymer can be either to inhibit one or more of the following processes during gas hydrate formation: nucleation or crystal growth. In addition the polymers have a scale inhibiting activity.
The polymers of this invention are preferably made by reacting polymers and copolymers of maleic anhydride with one or more amines containing 1-18 carbon atoms with or without added hydroxyamines, at a low enough temperature to prevent less water-soluble cyclic imide products from forming. The amine can be a monoamine or diamine. If one mole of amine is used per mole of maleic anhydride, the product has X=H. Although it is not necessary, these monocarboxylic products can be made more water-soluble by adding base such as NaOH. If two or more moles of alkylamine are used per mole of maleic anhydride, the product has X=RNH3. These products are more ionic, and therefore more water-soluble, than those formed using one mole of amine and no base. In addition, the R2NH3 ion also has some activity of its own in preventing hydrate formation, especially if R2 has 4-5 carbons.
Water-solubility can be increased by using maleic anhydride copolymers comprising comonomers having polar and/or ionic groups, by using less than 1 mole equivalent of alkylamine reacted with the maleic anhydride polymer, or by reacting a mixture of hydroxyamine and an alkylamine with the maleic anhydride polymer.
As mentioned above, the polymers of this invention are useful as additives for inhibiting the formation of gas hydrates in connection with hydrocarbon production and transportation.
The additives of the present invention may in addition to the polymers of the invention and other substances also contain a liquid or solid carrier or excipient. The amount of the polymers of this invention that has to be added is generally between 0.05 and 5 wt. %, preferably between 0.05 and 0.5 wt.%, based on the amount of water in the hydrocarbon-containing mixture. The polymers can be added to a stream of light hydrocarbons and water either as powders or preferably in concentrated solution.
The polymers of this invention can also be used together with various other substances, called synergists, to improve the overall performance of the product. These synergists are:
a) Polymers and copolymers of N-vinylcaprolactam, N-vinylpyrrolidone, alkylated vinylpyrrolidones, acryloylpyrrolidine, and polyamino acids such as polyaspartates.
b) Butoxyethanol and 2-butoxypropanol which can also be used as a solvent medium.
c) Tetrabutylammonium salts, tetrapentylammonium salts, tributylamine oxide, tripentylamine oxide and compounds containing the di- and trialkylammonium group, wherein the alkyl is particularly butyl or pentyl, and zwitterionic compounds having at least one butyl or pentyl group on the quaternary ammonium nitrogen atom, such as Bu3N+xe2x80x94CH2xe2x80x94COOxe2x80x94.
These synergists from classes a), b), and c) are preferably added in an amount of between 0.01 and 2.0 wt. % based on the water content.
An example of a synergist-containing product is formed by addition of 1 part of Gaffix VC713 (a terpolymer of N-vinyl caprolactam, N-vinyl pyrrolidone and dimethyiaminoethylacrylate) to 4 parts of the reaction product of xe2x80x9cGantrez AN-119-BFxe2x80x9d (a methyl vinyl ether-maleic anhydride copolymer) and isobutylamine.
The polymers of this invention can be formulated with a solvent such as water, a glycol or lower alcohol or a mixture of such solvents. Other production chemicals such as corrosion inhibitors, scale inhibitors and anti-foams can be formulated with the polymers of this invention. The polymers of this invention are also suspected to have anticorrosion and antiscaling properties of their own.
Particular preference is given to products which are formed by reacting a polymer which is built up from maleic anhydride and one or more substituted or unsubstituted olefins R3R4Cxe2x95x90CH2, with one or more acyclic C2-C18-diamines and, if desired, with one or more primary or secondary C1-C12-monoamines, where R3 and R4 are, independently of one another, hydrogen or a C1-C12-alkyl, C2-C12-alkenyl or C6-C12-aryl radical which may be interrupted by oxygen or xe2x80x94COxe2x80x94Oxe2x80x94 or xe2x80x94Oxe2x80x94COxe2x80x94 and R3 can also be xe2x80x94COOH.
The incorporation of the diamine makes it possible to prepare polymers which are water-soluble over a wide pH range, since simple reaction products of polymers based on maleic anhydride with aliphatic monoamines are polymers having carboxylate functions which become water-insoluble in an acid medium as a result of the protonation of the carboxylate groups and therefore precipitate from the aqueous solution. When suitable diamines are incorporated, the polymer takes on a cationic charge in the acid range and thus remains water-soluble.
These polymers can be alternating polymers of maleic anhydride and the corresponding olefin, as are formed, in particular, in low-pressure processes, or else random polymers having olefin: maleic anhydride molar ratios of  greater than 1 or  less than 1 which are formed predominantly in high-pressure processes. Preference is given to a molar ratio of olefin to maleic anhydride of 1:1 to 10:1.
Many of these polymers are commercially available or can be synthesized by a simple route. Thus, polymers of maleic anhydride and vinyl ethers are obtained under the name (copyright)Gantrez AN (ISP), (copyright)Gantrez ES (GAF), (copyright)Viscofras (ICI) or (copyright)Sokalan (BASF).
Polymers of maleic anhydride and the corresponding olefins are obtainable by methods known from the literature. A summary of these syntheses is given in Methoden der Organischen Chemie, Volume E 20 (Makromolekulare Stoff), pp. 1234-1250, Georg Thieme Verlag, Stuttgart, 1987.
The synthesis of alternating ethylene-maleic anhydride polymers and of random polymers of maleic anhydride and ethylene is described in the above reference and also in Polymer Science U.S.S.R. Vol. 25, No. 9, pp. 2151-2160, 1983.
The molecular weight of these polymers can vary within the range 1000- greater than 106 g/mol, but preference is given to molecular weights of 1000-40000 g/mol.
Diamine components which can be used are dialkyl-substituted diamines having 2 to 18 carbon atoms in the molecule, preferably having one primary and one tertiary amino group, e.g. N,N-diethylaminopropylamine, N,N-dimethylaminopropylamine, N,N-dipropylaminopropylamine and N,N-dibutylaminopropylamine. Preference is given to dialkyl-substituted diamines having 4 to 12 carbon atoms in the molecule; 3-dimethylamino-propylamine is very particularly suitable.
Suitable monoamine components are monoamines having a primary or secondary amino group and 1 to 12 carbon atoms in the molecule. Preference is given to amines of the formula R5NH2, where R5 is an unsubstituted, branched or unbranched alkyl radical having 1 to 12, preferably 1 to 5, carbon atoms. Examples are methylamine, ethylamine, propylamine, isopropylamine, n-butylamine, isobutylamine, the isomeric pentylamines and hexylamines, as well as octylamine and dodecylamine.
The polymers to be used according to the invention are prepared, for example, by reacting the polymer which is built up from maleic anhydride and one or more olefins with the above-mentioned monoamines and diamines in an aqueous or acqueous-alcoholic medium, the polymer being slowly introduced into the solution of the amines. Suitable alcoholic solvents are water-soluble mono-alcohols, e.g. methane, ethanol, propanoles, butanoles and. oxethylated monoalcohols as butyle glycol and butyle diglycol.
The sum of the molar amounts of the diamines and monoamines is 80-200% based on the anhydride content of the polymer. However, the diamines and monoamines are preferably added in such amounts that the sum of the amounts of diamines and monoamines corresponds to the anhydride content of the polymer. The molar ratio of diamine to monoamine is 100:0 to 10:90.
The reaction temperature selected can be from 0xc2x0 C. to the boiling point of the solvent, but is preferably selected so as to be below 50xc2x0 C. in order to make possible the formation of monoamide structures and to suppress ring closure reactions which form the cyclic imide. Clear solutions of the modified polymers are formed.
The above mentioned synergists include mixtures of polyamides with one or more different polymers having a carbon backbone and amide bonds in the side chains.
These include, in particular, polymers such as polyvinylpyrrolidone, polyvinylcaprolactam, polymers of vinylpyrrolidone and vinylcaprolactam, and also terpolymers of vinylpyrrolidone, vinylcaprolactam and further anionic, cationic and uncharged comonomers having a vinylic double bond, e.g. 1-olefins, N-alkylacrylamides, N-vinylacetamide, acrylamide, sodium 2-acrylamido-2-methyl-1-propanesulfonate (AMPS) or acrylic acid. Mixtures comprising homopolymers and copolymers of N,N-dialkylacrylamides such as N-acryloylpyrrolidine, N-acryloylmorpholine and N-acryloylpiperidine are also suitable. Likewise suitable are mixtures comprising alkylpolyglycosides, hydroxyethylcellulose, carboxymethylcellulose and other ionic or nonionic surfactant molecules. Particularly suitable mixtures are ones comprising quaternary ammonium salts, specifically tetrabutylammonium bromide and amine oxides such as tributylamine oxide.